This invention relates to swirling burners used with a gas turbine combustor, and, more particularly, to a premixing swirl burner suitable for use with a gas turbine combustor for performing combustion of a premix of fuel and air.
Oxides of nitrogen (NOx) released from various types of combustion facilities raise the problem of air pollution and environmental disruption. Particularly with regard to combustors for use with gas turbines, a program has been under way for diminishing generation of NOx by means of a dry process based on improvements of combustibility and a wet process using water and steam. However, in view of the fact that the generation of NOx in combustion is very complex, great difficulties are encountered in developing necessary technology and no decisively successful process has yet been developed and established as a new process for combustion. Particularly, although the dry process offers the advantage that no other medium is required (in the wet process, addition of water and steam is necessary), combustion by this process is under severe conditions of lean mixtures of fuel and air and low temperature. Thus, even if a reduction in NOx can be achieved to a certain extent, there is a concomitant rise if the amount of emission of CO. Generally the generation of NOx at the time of combustion is governed by the combustion gas of high temperature regions (of over 1800.degree. C.) locally developing in the combustion zone and caused by oxidation of the unburned nitrogen in the exhaust emissions and also the nitrogen in the air of combustion. These nitrogen oxides are referred to as thermal NO and fuel NO, and the former particularly depends on oxyen concentration and reaction time and is considerably influenced by gas temperature. Thus, if it is possible to realize low temperature combustion (of about 1500.degree. C.) in which no local high temperature regions exist, it would be possible to reduce NOx considerably in combustion.
Heretofore, it has been usual practice to use a system of combustion for reducing the generation of NOx for a gas turbine combustor in which excess air is introduced into the combustion zone to perform combustion with a lean mixture of fuel and air at low temperature. However some disadvantages are associated with this combustion system. In this combination system, difficulties are experienced in forming a uniform fuel-air mixture because combustion is sustained while air is mixed with fuel in diffusion mixing. In the process of combustion, enriched and lean areas tend to be formed in the fuel-air mixture and high temperature regions are formed as a result, so that it is impossible to achieve a marked reduction in NOx. Also, introduction of excess air results in supercooling of the flame surface, thereby causing an increase in the unburned components of the fuel, such as CO and rendering the combustion unstable. These phenomenons are largely accounted for by combustion of fuel-air mixtures of diffusion mixing. Thus, in order to obtain a dramatic reduction in NOx and CO in the exhaust emissions of combustion merely by changing the form of combustion, it is essential that a complete mixing of fuel and air be achieved before combustion to render the temperatures of flames uniform to avoid development of local high temperature regions in the mixture of fuel and air. This combustion system is generally referred to as a system of combustion of a premix of fuel and air in which fuel is mixed with air while being supplied to the combustor. In this premixing combustion system, the fuel-air mixture is combusted in an excess of air relative to the fuel, to thereby effectively reduce NOx and CO in the exhaust emissions.
The operation conditions for performing combustion by the above described premixing combustion system for the combustor of a gas turbine are as follows: combustion of a premix of fuel and air mainly takes place in the combustion chamber at the head of the combustor at partial load, and combustion of a fuel-air mixture at high load at or in the vicinity of the rated value takes place by burning a lean fuel-air mixture by simultaneously actuating a premixture swirling burner and a swing burner of the second stage located in the rear portion of the combustion chamber. Thus, the performances of the premixture swing burner and the swirling corner in the rear portion of the combustion chamber are important factors concerned in decoding the quality of combustion. In a gas turbine combustor, the range of operation from partial load to rated load is wide and any combustion would not be of any value unless the swirling burner shows a stable and excellent performance with respect to the ratio by weight of 0.004-0.018 of fuel to air. Moreover, the swirling burner should be of a type suitable for use in combustion at high flow velocity and high load and in a wide range of combustion, and has been in use as a flame protector for combustion of a fuel-air mixture of diffusion mixing at high load, such as in a gas turbine. However, in the combustion of a premix of fuel and air, there is considerable difficulty in controlling a change in the load over a wide range than the combustion of a mixture of fuel and air of diffusion mixing. Thus, if the load of a premix of fuel and air is increased to obtain full realization of advantages offered by the above described system in reducing NOx and CO in the exhaust emissions, the system would suffer the disadvantage that the region of partial load to be controlled would become wide in a gas turbine combustor, thereby reducing stability and reliability of a premixing burner as witnessed by a combustion loss of the burner head, for example. Thus, how to avoid a reduction in stability and reliability of combustion would be the problems that have to be one of avoided.